Field of the Invention
The present invention relates to PET resin powder capable of forming a coating film or paint film by close adhesion to the surface of a metal, or the like, and to a process of preparation. The present invention permits the use of small broken pieces of recovered containers such as PET bottles as raw PET resin material.
PET (polyethylene terephthalate) resin exhibits low adhesivity to the surface of metal. Moreover, even when a baked coated film is formed from the material, crystallization of the coated film progresses which results in the development of the tendency to craze or peel. Accordingly, the resin is normally not utilized as a powder coating (powder paint). In addition, PET resins and other thermoplastic resins generally have low heat resistance and are elastic. Consequently, when machine grinding is performed at ordinary temperatures, ground particles have strand-like or whisker-like projections exceeding 1 mm in length, making it impossible to handle the polymer as a so-called powder. Generally, the particle size of powder for electrostatic coating is required to be 5-150 .mu.m (average particle diameter 40 .mu.m) and that of powder for fluidization dip, 60-350 .mu.m (average particle diameter 150 .mu.m). Accordingly, it is very difficult to prepare a fine powder (5-350 .mu.m) suitable for powder coating, or the like, by means of ordinary machine grinding. Grinding is presently performed by refrigeration grinding method using liquid nitrogen. Refrigeration grinding requires a large quantity of liquid nitrogen, and it is also necessary to adapt grinding machines and other peripheral devices to specific applications which are capable of enduring low temperature environments. This necessitates the provision of large scale equipment with a resulting increase in grinding costs.